Process for vapor-liquid contacting and fractional distillation

ABSTRACT

A vapor-liquid contacting process is disclosed which may be used in gas treating, absorption or in fractional distillation. The process is characterized by the utilization of a liquid support plate (tray) having a flat vapor-liquid contacting area formed by uniformly spaced parallel members which provide long narrow passages for the rising vapor. The process is especially useful when a low pressure drop through the liquid support plate or a high tendency of the liquid support plate to prevent weeping is desired. The process allows higher operating capacity than the use of valve-type liquid support valves.

CROSS REFERENCE TO RELATED APPLICATION

This application is a continuation-in-part of my prior copendingapplication Ser. No. 182,355 filed on Aug. 29, 1980, now abandoned. Theentire teaching of my prior application is hereby incorporated byreference.

FIELD OF THE INVENTION

The invention relates in general to a process in which a rising vaporstream is intimately contacted with a liquid in an apparatus of specificdesign. The invention more specifically relates to such vapor-liquidcontacting steps as are performed in processes for fractionaldistillation and the absorption of specific components of a gas stream.The invention also relates to other vapor-liquid contacting processessuch as the scrubbing of gas streams for pollution control. The subjectgas-liquid contacting process utilizes a low pressure drop liquidsupport tray and therefore also relates to the design of vapor-liquidcontacting apparatus suitable for use as fractionation trays within afractional distillation column.

PRIOR ART

Vapor-liquid contacting processes are very well established commercialprocesses in which the art has reached a high level of sophistication.There therefore exists a large volume of accumulated knowledge as to thedesign and construction of fractionation trays. For instance, pages 3-25of Section 18 of the 4th Edition of the Chemical Engineers' Handbook,McGraw-Hill Book Co., 1963, describes several different types offractionation trays and presents a large amount of information useful inthe design of fractionation trays and the operation of fractionationcolumns.

One type of fractionation tray which is well described in the literatureis referred to as a grid tray. The vapor-liquid contacting area of thistray is often a single flat surface formed by a large number of spacedapart parallel horizontal strips or bars. The horizontal strips areseparated by a small distance to allow the vertical passage of fluidthrough the long narrow opening between adjacent strips. This type offractionation tray is described in U.S. Pat. Nos. 2,682,394; 2,711,308;2,750,174; 2,711,307; 2,860,860; 2,875,993; and 2,882,030. It isbelieved that these references are directed to grid trays in which boththe rising vapor and the descending liquid pass through the sameelongated slots between adjacent horizontal members. That is, thesereferences exemplify the use of a grid tray in a fractionation columnwhich does not have downcomers to carry the liquid to the next lowertray. The horizontal members forming the vapor-liquid contacting areasin the first three of these references may have sloping or curved sideswhich result in the members being widest at their upper surface.

U.S. Pat. No. 4,157,905 is believed pertinent for its teaching that gridtrays are recommended for use without a downcomer but could be used withdowncomers.

U.S. Pat. No. 2,747,849 illustrates a fractionation tray used withdowncomers in which the vapor-liquid contacting area has a substantiallyflat surface having a large number of elongated narrow openings throughwhich the rising vapor passes.

U.S. Pat. No. 3,592,452 presents a fluid contacting device which may beused as a fractionation tray. This device has an upper surface formed bya large number of spaced apart parallel and horizontal members whichextend across the column. A second set of horizontal closely spacedmembers having a different alignment supports the first set of parallelmembers. Either the upper or lower set of parallel members may berotated about a vertical axis to vary the available open area of thetray.

BRIEF SUMMARY OF THE INVENTION

The invention provides a vapor-liquid contacting process which isespecially useful when it is desired to achieve a very high contactingefficiency without incurring a substantial pressure drop through thecontacting zone. The process is characterized by the unique apparatusused to form the contacting zone. One embodiment of the invention may becharacterized as a process for separating chemical compounds byfractional distillation which comprises passing a vapor streamcomprising a first and a second chemical compound which each containless than ten carbon atoms per molecule upward through the vapor-liquidcontacting area of a plurality of horizontal fractionation trays mountedat different heights within a vertical containment vessel having asubstantially vertical inner surface, with each fractionation traycomprising a substantially flat horizontal vapor-liquid contacting areacomprising a plurality of horizontal parallel members having an uppersurface possessing a width of between about 0.15 cm and about 0.55 cmand being uniformly spaced apart by a distance between about 0.022 cmand about 0.080 cm and attached to a smaller second plurality ofhorizontal bars which are located below the parallel members, with thehorizontal bars being substantially perpendicular to the parallelmembers, and with adjacent parallel members of the vapor-liquidcontacting area having opposing surfaces which if extended upward wouldintersect at an angle between 5 and 120 degrees at a point above theupper surface of the vapor-liquid contacting area; and passing a liquidstream comprising the first and the second chemical compounds downwardthrough the containment vessel via a plurality of downcomers locatedwithin the containment vessel and operatively associated with thefractionation tray while the vessel is maintained at fractionationconditions which promote the enrichment of the vapor stream in the morevolatile of the first and second chemical compounds.

In other embodiments the horizontal members of the vapor-liquidcontacting area may have either flat or curved sides and are shaped suchthat the smallest distance between any two adjacent members is locatedat the upper surface of the members and is less than one-half thehorizontal distance separating the bottom of the adjacent parallelmembers.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is a simplified illustration of a fractionation column which maybe used to perform the subject process.

FIG. 2 is a cross-section of the fractionation column shown in FIG. 1taken along a horizontal plane at the point indicated in FIG. 1 andlooking downward toward the upper surface of one of the fractionationtrays in the column.

FIG. 3 is a cross-sectional diagram of the fractionation tray shown inFIG. 2 taken along a vertical plane at the point indicated in FIG. 2.

FIG. 4 is a cross-sectional diagram of a small portion of thevapor-liquid contacting area of the fractionation tray shown in FIG. 3.

FIG. 5 is a cross-sectional diagram of a small portion of a vapor-liquidcontacting area in which the parallel members 17 have curved sides.

DETAILED DESCRIPTION

Vapor-liquid contacting processes are in widespread use in the chemical,petrochemical and petroleum industries. For instance, vapor-liquidcontacting processes are used for the removal of certain compounds fromvapor streams, such as the scrubbing of sulfur dioxide from a flue gasstream. One of the most common vapor-liquid contacting processes is thefractional distillation process used in the separation of chemicalcompounds. These separations include the production of high purityoxygen or nitrogen by the cryogenic fractionation of liquefied air, theseparation of mixtures of water and various alcohols, the fractionationof light hydrocarbons and the fractionation of crude oil and variouspetroleum-derived fractions. The majority of the description herein isdirected to the use of the invention in a fractionation process.However, the subject process is a vapor-liquid contacting process ofgeneral application. It is therefor not intended to limit the scope ofthe invention by this mode of description.

The proper and effective design of an apparatus for use in avapor-liquid contacting process normally requires that a number ofimportant and sometimes conflicting design characteristics must beconsidered. These factors include but are not limited to the structuralstrength of the apparatus and its ability to support both its own weightand the weight of the liquid which rests upon it, the size, shape andnumber of the openings provided in the vapor-liquid contacting area ofthe tray, the efficiency of the tray, the tendency of the tray to"weep," the tendency of the tray to flood, and the pressure dropexperienced by the vapor rising through the tray. An ideal fractionationtray must also be resistant to plugging or breakage, and be easy toassemble from a number of smaller parts which are passed into an outervessel through the relatively small opening of a manway.

It is an objective of the subject invention to provide an effectivevapor-liquid contacting process. It is another objective of the subjectinvention to provide a fractionation process which operates at arelatively low pressure drop at a given upward vapor velocity. It is afurther objective of the subject invention to provide a fractionationprocess which will function efficiently over a wide range of operatingflow rates. Yet another objective of the subject invention is to providea low pressure drop process for treating vapor streams.

The subject invention provides many of the desired characteristics of afractionation process through the use of a contacting apparatus having aunique arrangement of several structural elements. At the hear of thisvapor-liquid contacting apparatus is a vapor-liquid contacting areaformed by a large number of closely spaced apart parallel and horizontalmembers. These parallel members are preferably of equal size and have awidth which is at least five times as great as the distance betweenadjacent members. The upper surface of these members forms thesubstantially flat upper surface of the vapor-liquid contacting area ofthe apparatus. These upper parallel members are attached, preferably bywelding in the case of metallic materials of construction, to a secondplurality of horizontal supporting bars or members which areperpendicular to the upper plurality of members. The lower members mayhave a depth which is several times that of the upper members to providean integrated structure having great rigidity and capable ofindependently withstanding the weight loadings experienced within afractionation column. There are fewer of the lower parallel members andthey are much more widely spaced apart than the upper parallel members.As used herein, the term "vapor-liquid contacting area" of afractionation tray is intended to refer to that portion of the surfacearea of the tray through which it is desired for vapor to pass upwardand which contains at least 95% of the total open area of thefractionation tray.

Referring now to FIG. 1, there is shown a simplified illustration of afractionation process deformed in a column 1 comprising severalfractionation trays 9 constructed in accordance with the subjectinvention. A feed stream is passed into an intermediate point of thiscolumn at a point now shown. The liquid which accumulates at the bottomof the column is withdrawn through line 5 and divided into a firstportion removed through line 8 as a bottoms product and a second portionwhich is passed through line 6 and an external reboiler 7 to generatevapor which is passed into a lower portion of the fractionation columnthrough line 6. An overhead vapor stream is removed from the column inline 4 and condensed by means not shown to form overhead liquid which isdivided into an overhead product stream withdrawn from the system and aliquid reflux stream which is returned to the column through line 3.Alternatively, the feed stream or a mixture of reflux liquid and thefeed stream may be passed into the column through line 3. Liquid fromline 3 flows horizontally across the uppermost fractionation tray. Itthen passes over the top of the weir 11 and enters a downcomer throughwhich it descends to the next lowest fractionation tray. The liquidphase present within the fractionation column thereby travels downwardin a sigmoid path traveling from tray to tray through the appropriatedowncomer associated with each tray. The vapors present within thefractionation column rise through the vapor-liquid contacting area ofthe fractionation trays and thereby flow in a generally countercurrentdirection to the descending liquid.

FIG. 2 is the view seen looking downward at an intermediate elevation inthe fractionation column toward the upper surface of a fractionationtray 9. In this view, it may be seen that the fractionation tray has acircular outer edge which abuts a large portion of the inner surface ofthe vertical containment vessel of the column. This provides a sealcapable of blocking fluid flow at the edge of the tray. This sealextends around the entire tray including the imperforate area under theinlet downcomer. The only point at which the tray is not adjacent theinner surface of the column is beyond the outlet weir 11. The chordalwall of the outlet downcomer of this fractionation tray is under theoutlet weir shown on the left-hand side of the figure. The vapor-liquidcontacting area of the tray is formed by five panels each comprising alarge plurality of parallel members which run across the upper surfaceof the tray parallel to the chordal downcomer walls and perpendicular toa line drawn between the central vertical axis of the inlet and outletdowncomers. The individual panels rest upon the imperforate supports 12,with the seam between the ends of the panels and the supports beingcovered by the long narrow cover plates 13. The edges of the tray mayrest on a flat strip welded to the inner surface of the column and whichhelps form the desired seal.

FIG. 3 is an enlarged cross-sectional view of the fractionation tray 9shown in FIG. 2. An inlet downcomer wall 10 is present at the right-handside of this figure. One horizontal imperforate support 12 attached tothe inner surface of the vertical wall of the column 1 is locatedimmediately below the inlet downcomer. This support has a channeled lip14 for added structural strength. On the left-hand side of the figure,there is shown a second imperforate support 12 having a similar lipwhich is attached to the outlet downcomer. The wall 10 of the outletdowncomer extends upward to an upper edge which acts as the outlet weir11 of the fractionation tray and retains a minimum liquid level upon theupper surface of the tray. The vapor-liquid contacting area of the trayis formed by the perforate panels which rest upon and extend between thesupports 12. With a proper upward vapor flow the panels and the supportsact as a unitary liquid support plate which retains the liquid flowingacross the tray. Vapor rises through the elongated openings between theupper parallel wires or members 16 which are supported by the lowermembers 15. The ends of the lower members rest on the supports, with theseam between the panels and the supports being covered by a plate 13which is bolted to the supports to hold the panels in position.

FIG. 4 presents details of the preferred structure of the panels formingthe vapor-liquid contacting area of the tray shown in the precedingfigure. The upper parallel members 16 are perpendicular to the lowersupport members or bars 15. The lower members may be round, wedge-shapedor rectangular in cross-section. The upper members have a wedge-shapedcross-section such that the opposing vertical surfaces of adjacentmembers would intersect at an angle alpha if extended above the surfaceof the tray. This angle is preferably between 10° and 90°. The smalldistance "d" and the much larger distance "w," which is referred toherein as the width of the upper member, are also indicated on thisfigure.

FIG. 5 shows an alternative construction of the vapor-liquid contactingarea of the subject apparatus. In this embodiment, the vertical sides 19of the upper members 17 are curved rather than flat. The other aspectsof this construction are the same as in FIG. 4. The distance between twoadjacent upper members "d" is smallest at the flat upper surface of themembers. The space "s" at the bottom of these members is at least twiceas large as the distance "d" and may be 3 to 8 times greater in length.The elongated opening therefore tapers to its smallest width at theupper surface of the vapor-liquid contacting area. The upper members areattached to horizontal support members 18, which are perpendicular tothe upper horizontal members.

The vapor-liquid contacting area of the subject apparatus is preferablyformed by a grid of perpendicular metal wires which are welded togetherto form a high strength screen. This grid is flat and is mounted in asubstantially horizontal position as part of the apparatus. The grid hasan upper surface formed by the upper layer of wires or members, whichare all located at the same elevation. The wires of the upper layer areparallel to each other and separated by only a small distance.Preferably the upper surface of each of the upper members is flat andhorizontal such that the upper surface of the grid is flat anduninterrupted except for the openings between adjacent bars. Theparallel members of the upper layer are perpendicular to the parallelmembers of the lower layer, which are in the same plane as the upperlayer but much more widely spaced apart. The upper and lower layers areattached at each point the individual members intersect, with the lowermembers functioning as the support bars and connecting means for theupper members. It is believed the lower members do not affect thevapor-liquid contacting action of the apparatus. The lower supportmembers will normally be larger in cross-sectional size and fewer innumber than the upper members. Normally, there will be at least threetimes as many upper members as lower members. It is greatly preferredthat the perpendicular members are welded to each other but they may beattached in other ways, especially if nonmetallic members are used inthe apparatus. The members may be formed from carbon or stainlesssteels, more corrosion-resistant alloys or various plastics andpolymeric materials.

Welded grips of perpendicular bars are in widespread commercial use.Some of the typical applications of these structures are water wellintake screens, catalyst retention screens in catalytic reactors andfluid collection or distribution assemblies. This grid is often weldedtogether in the form of a cylindrical gridwork by wrapping a continuouswire over longitudinal bars. This gridwork is then cut lengthwise andflattened to form the finished grid. For instance, in U.S. Pat. No.2,046,456, there is shown the use of a cylindrical porous wall of thistype of construction as a well point and well screen. A centrifugalfluid strainer using a cylindrical grid is shown in U.S. Pat. No.3,481,474. A screening unit for use in the mining industries whichincludes a flat porous grid of the preferred construction is shown inU.S. Pat. No. 3,483,974. This porous grid may have a structure andconfiguration very similar to that employed in the vapor-contacting areaof the subject apparatus. U.S. Pat. No. 3,584,685 presents a strainerusing a welded cylindrical filter element. U.S. Pat. No. 3,667,615illustrates many of the different welded grids which may be formed usingbars of different cross-section shape. U.S. Pat. No. 4,170,626 presentsa gas distribution device for use in fluidized beds of solid particlescomprising a cylindrical porous wall formed of a grid having thepreferred construction.

The upper members preferably have flat vertical sides similar to thoseshown in FIG. 4 but may have different shapes. In the embodiment of FIG.4, the opposing surfaces of adjacent bars are inclined from truevertical such that the elongated opening between the adjacent bars formsa "V" slot having its broadest opening at the bottom. An alternativemethod of describing this configuration is that the opposing surfaces ofadjacent members would intersect at an angle between 5 and 120 degreesif extended above the upper surface of the grid. Preferably, this angleis between 10 and 90 degrees.

The above cited references, which described various uses of welded wiregrids, also describe grids of various types including those having a "V"slot opening. This opening configuration is often credited with beingbeneficial to the operation of the different apparatus, such as wellscreens, as it tends to be self cleaning. The "V" slot is characterizedas self-cleaning since any particles or movable deposits which pass intothe smallest part of the slot will be removed from the screen by theflowing fluid. This is because in the applications described in thesereferences the screens are oriented such that the fluid flows inwardthrough the smallest opening of the V-slot and then through the screen.

In the subject process the only desired fluid flow is upward through thecontacting area. The direction of the fluid flow through the contactingarea of the subject process is therefore believed to be opposite thatused in the previously described filtration or collection devices whichhave screens with self-cleaning V slots. In the subject process thefluid flows upward into the largest part of the slot and then towardsthe smallest part of the slot where it exits. This flow path is thoughtto be at least partially responsible for the low pressure drop and lowweepage which is observed in the subject process. The rising vapors arechanneled into the smallest part of the openings. This increasesvelocity effects which tend to support the liquid on the upper surfaceof the contacting area rather than allowing the liquid to flow downward.

The low pressure drops through the apparatus used in the subject processmay simply result from the straight vapor flow path which is availableto the rising vapors. That is, the vapors do not have to changedirection during passage upward through the vapor-liquid contacting areaor to support any valve mechanism. The momentum lost in directionchanges results in a requirement for a higher pressure to achieve thesame flow rate. The apparatus used in the subject process does notrequire the rising vapors to change direction, and the only pressuredrop encountered is that required to force the vapor upward through thetray openings. The apparatus therefore apparently does not have theadditional "parasitic" pressure drop encountered in valve or "ballast"trays.

The advantages to the use of the subject process are apparent in theresults of a comparison study between processing conditions resultingfrom the use of a commercial type valve tray and a tray as describedherein and referred to as a screen tray. The various tests wereperformed using a 3-foot diameter column having 5 trays at a 13-inchspacing except for the pressure drop tests of the valve trays which wereperformed using three trays. The screen tray had a flat upper surfacecomposed of tapered members similar in cross section to those shown inFIG. 4. The width "w" of each member was 0.120 inch (0.305 cm) and thedistance between adjacent members was 0.02 inch (0.051 cm). The screentray had an open area of about 0.51 sq. feet or about 9.5 percent of thetray's area. The valve tray had an open area of about 0.68 sq. feet orabout 11 percent of its area. The valve tray was similar in constructionto that shown in U.S. Pat. No. 3,080,155 and had openings about 1 -17/32inches in diameter.

Based on this comparison of one specific type of screen tray and onespecific type of valve tray, it appears that the efficiency and theheight of an equivalent unit are similar for the two types of trays.However, the screen tray has a significantly lower pressure drop andabout 25 to 40 percent greater capacity. A process using a screen traymay therefore operate at a much higher capacity than a process usingvalve trays of the same area. The subject invention can be used toparticular advantage in increasing the capacity of existingfractionation columns.

A total of sixty-eight tests were run measuring the pressure dropcharacteristics of the two types of trays. In addition a total of ninetytests were run to measure the efficiency of the two trays. In thesetests both the flow rate of the rising vapor stream and the flow rate ofthe liquid passing downward through the column were varied as is normalin experiments. The actual rates of flow of these streams usuallydiffered from the target rates of flow, and the tests were conductedwhen the equipment was lined-out at the approximate target rates offlow. This results in the comparison of the two types of trays being atsimilar but not identical conditions. Because of this fact and the largenumber of tests, it is believed the results of the tests are bestcompared after a reduction of the experimental data to graphical form.The following tables present the values of some of the curves achievedin this manner as read from the graphs.

Tables 1 and 2 compare the pressure drop of the two types of trays, withTable 1 giving the values for no liquid being present in the column andTable 2 for a liquid flow rate through the column of 100 gallons perminute (gpm). The much lower pressure drop of the screen tray ascompared to the valve tray is evident by an examination of either table.The pressure drop advantage of the screen tray increases with the vaporrate, which is expressed in terms of a "C" factor. The "C" factor is thevolumetric rate of vapor flow upward through the tray expressed in termsof standard cubic feet per second of vapor per square foot of tray area.The values listed for the pressure drop are in inches of water. FromTable 2 it may be seen that the average pressure drop of the screen trayis only about 43 percent of that for the valve tray.

Tables 3 and 4 present data obtained in efficiency tests of the twotypes of trays. In these tests a very dilute aqueous solution of sodiumhydroxide was used to scrub carbon dioxide from a rising air stream. Thefeed vapor contained about 1.0 percent carbon dioxide and the feedliquid contained about 4.0 percent NaOH by weight. The change incondition of the vapor and liquid streams was used to calculate theefficiency of the trays. The results are reported in terms of a KGAfactor having units of lb-mole CO₂ per cubic foot-hour-atmosphere. Table3 shows results at a liquid flow rate of 100 gpm while Table 4 listsresults at a liquid flow rate of 200 gpm. No data is available in sometests. Most of these are in the high vapor rate tests and are due tolarge pressure drops through the valve tray which resulted in a decisionnot to attempt tests at those conditions. At equivalent conditions theKGA factor for the valve tray was uniformly above that of the screentray by a small margin reaching 19%. However, it may also be seen thatthe low pressure drop characteristics of the screen tray allowed itsoperation at higher vapor rates (C factors) which resulted in evenhigher KGA factors being achievable.

Tables 5 and 6 list the height of a theoretical unit (HTU) in feetcalculated from the test results versus the vapor rate or C factor forthe two types of trays. A lower HTU is normally preferred. The HTU ofthe valve tray was less than that of the screen tray, but not by greatpercentages.

At a vapor flow rate or C factor of 0.2 cubic feet per second per eachsquare foot of tray, the dry pressure through the valve tray was about4.7 inches of water and all of the liquid loaded pressure drops weresignificantly greater. For instance, the pressure drop at 20 gpm isabout 6.0 inches and a liquid flow rate of 100 gpm produced a pressuredrop of about 6.5 inches. In comparison at this vapor flow rate thepressure drop through the screen tray at a 200 gpm liquid flow rate, thehighest liquid rate used, is about 3.2 inches. The subject tray ispreferably used in a process having high vapor flow rates, with thevapor rate being at least above 0.18 cubic feet per second per squarefoot of tray area. It is especially preferred that this vapor rate isabove 0.20 cubic feet per second per square foot of tray. It is alsopreferred that the operating conditions of the process include apressure drop of less than 4 inches of water. A further preference isfor a liquid flow rate between about 5 and about 26 gallons of liquidper square foot of tray area.

                  TABLE 1                                                         ______________________________________                                        (dry)                                                                                      Pressure Drop                                                    C              screen  valve                                                  factor         tray    tray                                                   ______________________________________                                        0.141          0.4     2.4                                                    0.200          0.8     4.7                                                    0.245          1.3     7.0                                                    0.283          1.7     *                                                      0.316          2.2     *                                                      ______________________________________                                         *no data                                                                 

                  TABLE 2                                                         ______________________________________                                        (100 gpm)                                                                                  Pressure Drop                                                    C              screen  valve                                                  factor         tray    tray                                                   ______________________________________                                        0.141          2.6     4.5                                                    0.200          2.8     6.5                                                    0.245          3.1     8.8                                                    ______________________________________                                    

                  TABLE 3                                                         ______________________________________                                                     KGA @ 100 gpm                                                    C              screen  valve                                                  factor         tray    tray                                                   ______________________________________                                        0.10           1.4     1.6                                                    0.15           1.6     1.9                                                    0.20           1.9     2.2                                                    0.25           2.2     *                                                      ______________________________________                                         *no data                                                                 

                  TABLE 4                                                         ______________________________________                                                     KGA @ 200 gpm                                                    C              screen  valve                                                  factor         tray    tray                                                   ______________________________________                                        0.05           *       1.7                                                    0.10           1.9     2.0                                                    0.15           2.2     *                                                      0.20           2.5     *                                                      ______________________________________                                         *no data                                                                 

                  TABLE 5                                                         ______________________________________                                                     HTU @ 100 gpm                                                    C              screen  valve                                                  factor         tray    tray                                                   ______________________________________                                        0.10           18.8    17.0                                                   0.15           23.8    23.8                                                   0.20           28.6    26.0                                                   0.25           34.0    *                                                      ______________________________________                                         *no data                                                                 

                  TABLE 6                                                         ______________________________________                                                     HTU @ 50 gpm                                                     C              screen  valve                                                  factor         tray    tray                                                   ______________________________________                                        0.05           *       11.6                                                   0.10           24.0    21.0                                                   0.15           30.0    28.2                                                   0.20           36.0    30.6                                                   0.25           42.4    *                                                      ______________________________________                                         *no data                                                                 

The desired tapered fluid passageway or opening between adjacent membersmay be a shape different than the preferred "V" slot. The shape of thiselongated opening is determined by the shape of the upper members. Theupper members can have a wide variety of shapes other than that shown inFIG. 4. For instance, in FIG. 5 these same members have curved opposingvertical sides. Another alternative is for the opposing sides to havedifferent shapes or inclinations. As one example of this, the uppermembers could have a cross-section in the shape of a right triangle andbe fastened in place so that they have a horizontal upper surface, atruly vertical side and a single inclined side. The vertical sides ofthe members are preferably inclined from true vertical. Unless otherwisespecified, any reference herein to the sides or vertical sides of themembers is intended to refer to the flat or curved inclined surfaceswhich extend downward from the upper surface of the members. Anyreference herein to opposing sides is intended to refer to the adjacentsides of two different adjacent members.

The elongated parallel members which form the upper surface of thevapor-liquid contacting area are spaced apart by relatively smalldistances compared to the opening present in most commonly usedfractionation trays. The distance between adjoining members, which islabeled "d" in FIG. 4 is between 0.010 and 0.080 cm. Preferably, thisdistance is between 0.022 and 0.051 cm. Unless otherwise specified, anyreference to the spacing or distance between the parallel members isintended to indicate the minimum distance between adjacent members,which is preferably at the top of the members. The width of theuppermost portion of these members, labeled as "w" in FIG. 4 ispreferably between about 0.15 and 0.55 cm. These upper members arepreferably made of stainless steel or other non-corroding material sinceany corrosion would tend to reduce the already small space between theadjacent upper members.

It is believed that because this space is small the surface tension ofthe liquid tends to suspend the liquid above the opening therebyresulting in the low tendency of the apparatus to "weep" infractionation tray simulations. One of the most important factors indetermining the distance "d" and the width "w" of the upper members isthe total open area which is to be provided in the vapor-liquidcontacting area. This open area should be between 5.0 and 16.5 percentof the total surface area of the vapor-liquid contacting area.Preferably, the open area of the subject apparatus is between 7.5 and13.5 percent when used as a fractionation tray. As used herein, the term"open area" is intended to refer to the percent of the total area of thetray, exclusive of downcomer areas, which is intentionally left open bythe provisions of perforations or openings for the upward passage of thevapor or liquid. The number and dimensions of the lower perpendicularmembers is set by the expected total weight loading on the vapor-liquidcontacting area and the size of individual sections of the contactingarea which are self-supporting.

The vapor-liquid contacting area of the apparatus is preferablyassembled from rectangular panels having a maximum size of about 3meters by about 0.45 meters. These panels may be passed into thecontainment vessel through an available opening such as a manway andthen laid in place on supports attached to the inner surface of thevessel and extending across its cross-section. The panels are thenbolted or clamped down to prevent movement. The number and size of thepanels will be set by the diameter of the containment vessel.

One embodiment of the invention may be characterized as a vapor-liquidcontacting process which comprises the steps of passing a vapor streamupward through a liquid support plate located within a containmentvessel having a substantially vertical inner surface, with the liquidsupport plate having an upper surface and an outer edge which isadjacent to at least a portion of the inner surface of the sidewall ofthe vapor containment vessel, with the liquid support plate having asubstantially horizontal vapor-liquid contacting area comprising aplurality of horizontal parallel members having an upper surfacepossessing a width of between about 0.15 and about 0.55 cm and beinguniformly spaced apart by a distance between about 0.022 and about 0.080cm, with the vapor-liquid contacting area of the support plate having anopen area between about 5.0 and about 16.5 percent and also having aflat upper surface, and with the parallel members being shaped such thatthe smallest distance between any two adjacent parallel members islocated near the upper surface of the adjacent parallel members and thedistance between the vertical sides of adjacent parallel members is atleast twice as large at the bottom of the adjacent parallel members asat the top of adjacent parallel members; and passing a liquid streamacross the vapor-liquid contacting area of the liquid support plate in adirection which is substantially perpendicular to the parallel membersof the vapor-liquid contacting area while the liquid and the vaporstreams are maintained at contacting conditions.

A typical fractionation tray comprises an imperforate horizontal arealocated under the inlet downcomer. This is a liquid receiving area andis part of the "dead area" of the tray through which it is intended forno liquid or vapor to pass. This liquid receiving area may be separatedfrom the vapor-liquid contacting area by a vertical inlet weir, but theuse of such a weir is not preferred. Inlet weirs are normally only usedwhen liquid rates are expected to be low enough to allow vapor to risethrough the column if the weir is not present. Additional amounts ofdead area are normally located at the periphery of the tray especiallyat points where it rests on a support ring or lip fastened to the innersurface of the tray. An outlet weir, which is normally formed by anupward extension of the outlet downcomer wall, is commonly present onthe upper surface of the tray. The height of the outlet weir is set bythe static liquid level which is to be present on the tray. The inletand outlet downcomers are often vertical chordal walls as this is asimple structure to fabricate and also serves to both collect anddischarge the descending liquid in a manner which promotes uniformliquid flow across the tray. It is preferred that no flow directingvanes are placed on the tray, although these and other flow distributionmeans such as specially designed inlet and outlet weirs could be used onthe subject apparatus if it is quite large in diameter. The tray orother contacting apparatus is preferably horizontal, although the traymay be slightly tilted toward the liquid outlet to reduce the gradientrequired for the desired liquid flow rate across the surface of thetray. As used herein, the term "substantially horizontal" is intended toindicate an inclination from true horizontal of less than 5 degrees.

The horizontal portion of the total apparatus is located within acontainment vessel. This vessel will have a vertical inner surface if itis part of a fractionation column, but the inner surface could be slopedif the apparatus is used for other purposes. As used herein, the term"substantially vertical" is intended to indicate an inclination fromtrue vertical of less than 5 degrees.

The containment vessel will normally be entirely enclosed except for thenecessary transfer conduits for entering and exiting fluid streams,control system sensors, etc. Fractionation columns are normallyfabricated from metals such as carbon or stainless steel. Highlycorrosive compounds may dictate the use of more exotic metals. Thecontainment vessels of other types of contacting apparatus, such as fluegas scrubbing towers, may be made from other materials includingconcrete or fiber reinforced polyester or other plastics.

The preferred mode of the subject invention is as a process for theseparation of two or more different chemical compounds by fractionaldistillation. The fractionation column used in this process would beoperated in accordance with normal operating procedures. The conditionsused during the operation of fractionation processes are normally set bysuch factors as the temperature and pressure required to produce bothvaporous and liquid phases containing the two chemicals compounds whichare being separated. Other limitations are set by the tendency ofcertain compounds, typically aromatic or olefinic hydrocarbons, tothermally degrade into undesired compounds when exposed to hightemperatures in a reboiler. A broad range of fractionation conditionsincludes a bottoms pressure between about 0.1 atmosphere absolute andabout 35 atmospheres and a bottoms temperature between 0° and about 350°C. The preferred operating conditions include a superatmosphericpressure between about 1.0 and 12 atmospheres absolute and a temperatureabove 65° C. Cryogenic separations of various normally gaseousmaterials, such as oxygen, nitrogen and carbon dioxide, can be conductedat much lower temperatures including temperatures of about minus 150° C.or below. A reflux ratio between about 1.0:1.0 and about 5.0:1.0 may beemployed, and the column will normally contain from 10 to about 90 ormore trays.

The subject process is not limited to use in fractional distillationprocesses. It may be used in a wide variety of vapor-liquid contactingprocesses including pollution control and solvent recovery processes.One example of this is the scrubbing of power plant flue gases to reducesulfur oxide emissions or to recover carbon dioxide, a situation inwhich a very low pressure drop through the contacting apparatus isdesired. The process may also be utilized in absorption columns, such asthose used to remove hydrogen sulfide or heavy hydrocarbons from streamsof vaporous light hydrocarbons. It is contemplated that the subjectapparatus and process could also be utilized to perform separationsusing liquid-liquid extraction with a minimal amount of structuralchange.

I claim as my invention:
 1. A vapor-liquid contacting process whichcomprises:(a) passing a vapor stream upward through a liquid supportplate located within a containment vessel having a substantiallyvertical inner surface, with the liquid support plate having an uppersurface and an outer edge which is adjacent to at least a portion of theinner surface of the sidewall of the vapor containment vessel, theliquid support plate having a substantially horizontal vapor-liquidcontacting area comprising a plurality of horizontal parallel membershaving an upper surface possessing a width of between about 0.15 andabout 0.55 cm and being uniformly spaced apart by a distance betweenabout 0.022 and about 0.080 cm, and with the parallel members beingshaped such that the smallest distance between any two adjacent parallelmembers is located at the upper surface of the adjacent parallel membersand the distance between the vertically sloping sides of adjacentparallel members is at least twice as large at the bottom of theadjacent parallel members as at the top of adjacent parallel members;and, (b) passing a liquid stream across the vapor-liquid contacting areaof the liquid support plate in a direction which is substantiallyperpendicular to the parallel members of the vapor-liquid contactingarea while the liquid and the vapor streams are maintained at contactingconditions.
 2. The process of claim 1 further characterized in that thevapor stream and the liquid stream both comprise hydrocarbonaceouscompounds.
 3. The process of claim 2 further characterized in that theliquid stream is transported downward through the containment vesselthrough downcomer means operatively associated with each liquid supportplate.
 4. The process of claim 3 further characterized in that theparallel members of the vapor-liquid contacting area have a flat uppersurface and flat vertically sloping sides.
 5. The process of claim 1further characterized in that the contacting conditions include apressure drop through the liquid support plate of less than four inchesof water and a vapor flow rate upward through the contacting areagreater than about 0.20 cubic feet per second per square foot of tray.6. A process for separating chemical compounds by fractionaldistillation which comprises:(a) passing a vapor stream comprising afirst and a second chemical compound upward through the vapor-liquidcontacting area of a plurality of horizontal fractionation trays mountedat different heights within a vertical containment vessel having asubstantially vertical inner surface, with each fractionation traycomprising a substantially flat horizontal vapor-liquid contacting areacomprising a plurality of horizontal parallel members having an uppersurface possessing a width of between about 0.15 and about 0.55 cm andbeing uniformly spaced apart by a distance between about 0.022 and about0.080 cm and attached to a smaller second plurality of horizontal barswhich are located below the parallel members, with the horizontal barsbeing substantially perpendicular to the parallel members and withadjacent parallel members of the vapor-liquid contacting area havingopposing surfaces which if extended upward would intersect at an anglebetween 5 and 120 degrees at a point above the upper surface of thevapor-liquid contacting area; and, (b) passing a liquid streamcomprising the first and the second chemical compounds downward throughthe containment vessel via a plurality of downcomers located within thecontainment vessel and operatively associated with the fractionationtray while the vessel is maintained at fractionation conditions whichpromote the enrichment of the vapor stream in the more volatile of thefirst and second chemical compounds.
 7. The process of claim 6 furthercharacterized in that the vapor stream is passed upward through thecontacting area of the trays at a rate greater than about 0.18 cubicfeet per second per square foot of tray and in that the pressure dropacross each fractionation tray is less than about 4 inches of water. 8.The process of claim 7 further characterized in that the flow rate ofthe liquid stream across the fractionation tray is between about 5 andabout 26 gallons per minute per square foot.
 9. The process of claim 6further characterized in that the parallel members of the vapor-liquidcontacting area have a flat upper surface.
 10. The process of claim 6further characterized in that the first and the second chemicalcompounds are hydrocarbonaceous compounds containing less than tencarbon atoms per molecule.
 11. The process of claim 10 furthercharacterized in that the open area of the vapor-liquid contacting areaof the fractionation trays is between about 7.5 and 13.5 percent. 12.The process of claim 6 further characterized in that the vapor stream ispassed upward through the contacting area of the trays at a rate greaterthan 0.2 cubic feet of vapor per second per square foot of tray.